Triassic to Neogene tectono-magmatic events within Lorne Basin evolution, coastal New South Wales, eastern Australia
Within the Lorne Basin, Mesozoic granitoid plutons and dacitic to rhyolitic extrusions form extensive components in a faulted, eroded Triassic sedimentary basin. Late Cenozoic plugs and flows of evolved basalts represent easterly extensions of the eroded Comboyne Volcano. The basin marks a terminal outpost of the New England Orogen, much modified by later alkaline felsic intrusions and flow remnants. Many origins are invoked for its tectonic genesis and magmatic evolution including plate tectonic, caldera and even meteoritic impact events. This review uses extended field study, zircon fission track, K–Ar and Ar–Ar age-dating, petrography and geochemical analyses on igneous suites. These results are combined with previously published and unpublished studies to clarify relationships between volcanic and plutonic suites, structural features and paleotopographic developments, allowing for an enlarged synthesis of Lorne Basin magmatism and its tectonic contexts. Dating reveals rhyodacitic volcanism began at ca 221 Ma (Milligans Road) and culminated in emplacements of I- and A-type dioritic to granitic suites (North Brother, Middle Brother) at ca 217–212 Ma (MAGMATIC EPISODE 1, late Triassic). Voluminous alkaline rhyolitic feeders and flows (Bago Range, Batar Creek) erupted between ca 187 and 160 Ma (MAGMATIC EPISODE 2, early–mid-Jurassic). Lavas infilled extended drainages cut into a dissected, faulted terrain and disrupted former drainage patterns. An alkaline monzodioritic to granitic pluton and late-stage rhyolitic dyke swarm (South Brother) were emplaced at ca 126 and 119 Ma (MAGMATIC EPISODE 3, early Cretaceous). This chemically distinct magmatism coincided with thermal coastal faulting along eastern Australia. A large eroded plug and dyke complex (Mount Lorne) shows steep flow banding and lies within dissected radial flow infillings. This prominent eruptive conduit tapped a significant ring fault system. Although undated, its petrography resembles features related to MAGMATIC EPISODE 3. The study revealed a far greater profusion of rhyolitic feeders and flows than previously thought, making likely volcanic volumes within episodes difficult to estimate. The youngest volcanic remnants infill later drainages, are dated between ca 17 and 13 Ma (MAGMATIC EPISODE 4, mid-Neogene), and are evolved derivatives related to the adjacent Comboyne central volcano. Some evolved basalts contain feldspathic and mafic crystallisations indicating mid-crustal melt processes. These bimodal suites represent a traverse over an asthenospheric plume during northward lithospheric migration of eastern Australia. Minor vitrophyric bodies recorded across the Lorne Basin only intrude Carboniferous basement beds. Their origin and age are enigmatic and represent a final puzzle to solve in the evolution of Lorne Basin magmatism.